1. Technical Field
The present invention relates to a screen, a rear projector, and an image display apparatus.
2. Related Art
In recent years, a projector has come into wide use. In addition to a front projection type projector that is used mainly for business presentation, there is recently growing the recognition of a rear projection type projector as a form of a large-sized television (PTV: projection television). The biggest advantage of a projection type display apparatus is that the projection type display apparatus can provide a screen having the same size as direct view type displays, such as a liquid crystal television or a PDP, with a low cost as compared with the direct view type displays. However, as the direct view type displays are also becoming cheap, high resolution and performance are requested even to projection type display apparatuses.
A projector illuminates light emitted from a light source, such as an arc lamp, onto a light modulation element, such as a liquid crystal light valve and projects projected light modulated by the light modulation element onto a screen, such that an image is displayed on the screen. At this time, while an image is being displayed on the screen, the entire surface of the screen glares. This occurs due to brightness unevenness according to interference of light beams and is called spectral noises, so-called scintillation.
Here, a principle of occurrence of the scintillation will be described.
As shown in FIGS. 18A and 18B, light beams emitted from a light source 70 are transmitted through a liquid crystal light valve and are then projected onto a screen 74. The projected light beams projected onto the screen 74 are diffracted by scattering components 72 contained in the screen 74. Then, the diffracted light beams move like secondary wave sources to be diffused. As shown in FIG. 18B, two spherical waves of the secondary wave sources strengthen or weaken each other depending on the phase relationship between the two waves. As a result, the spherical waves appear as bright and dark fringe patterns (interference fringes) between the screen 74 and a viewer. When eyes of the viewer focus on an image surface S on which the interference fringes occur, the viewer recognizes the interference fringes as scintillation that causes the screen to glare.
The scintillation gives a viewer, who desires to see an image formed on the screen surface, an unpleasant feeling as if a veil, a lace cloth, or a cobweb exists between the screen surface and the viewer. In addition, since the viewer sees double images including an image on the screen and the scintillation, the eyes of the viewer desire to focus on both the images, which causes the viewer to feel fatigued. Accordingly, the scintillation causes the viewer to be so stressful that the viewer cannot stand.
In recent years, a new light source that will substitute for a known high-pressure mercury lamp is under development. In particular, expectation on a laser light source serving as a next-generation light source for a projector is increasing in terms of energy efficiency, color reproduction, long life, instantaneous lighting, and the like. However, light beams that are projected onto a screen by a laser light source have very high coherency because phases of light beams in adjacent regions are equal to each other. The coherent length of the laser light source may be several tens of meters. Accordingly, in the case when light beams emitted from the same light source are split and are then combined again, light beams that are combined through an optical path difference shorter than the coherent length cause strong interference. As a result, scintillation (interference fringes) occurs more definitely than a case of the high-pressure mercury lamp.
For this reason, a technique for reducing the scintillation is essential particularly in making a projector using a laser light source.
The following techniques are disclosed as measures for reducing the scintillation.
JP-A-11-038512 discloses a screen having a three-layered structure including a diffusion layer, a transparent (lenticular lens), and a diffusion layer, thereby optimizing a diffusing property of the screen. Thus, in the case when a scattering layer is complicated, the random property of interference spots increases. For this reason, if fine components (interference fringes having small spatial frequencies) of the spots increase, there may occur an effect that light beams are integrated and averaged due to an afterimage characteristic of human eyes when the eyes are averted. In particular, since the eyes are frequently averted in the case of watching moving pictures, reduction of scintillation may be expected.
JP-A-2001-100316 discloses a screen in which light beams, electric field, magnetic field, heat, stress, and the like are applied to a light scattering layer such that the shape, relative position, or refractive index of a light scattering body contained in the light diffusion layer change with time. Thus, by causing the scattering distribution or phases of scattered waves to change with time by the use of the light diffusion layer, it may be possible to expect occurrence of the scintillation to be prevented.
However, in JP-A-11-38512,a scattering state of a last scattering surface is fixed. Accordingly, the phase distribution of a space between a viewer and a screen, on which interference between light beams generated from points on a scattering surface occurs, is also fixed. As a result, an interference spot is also viewed as a fixed image. That is, the interference spots do not completely disappear. In addition, the screen is not effective particularly in a projector including a laser light source that is highly coherent. In addition, in the configuration using the high scattering property, an image may be defocused. Accordingly, an original purpose for realizing a high-quality image cannot be solved.
Further, in JP-A-2001-100316,a great amount of driving energy is needed to change the shape, relative position, or refractive index of a light scattering body. Furthermore, in the case of using the driving units, the efficiency of transmission of energy to the scattering layers is low and vibration, noise, unnecessary electromagnetic waves, and exhaust heat occur, which may disturb a viewer who wants a pleasant watching environment. Moreover, in the configuration in which the light scattering layer moves in the focusing direction, the size of an image changes. Accordingly, an outline of the image in the horizontal direction also changes, which causes an image to be defocused.